[Xen-devel] Re: [rfc] [patch] grant_entry.flags accessors

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 27 Jun 2006, at 16:06, Hollis Blanchard wrote:

> In the couple cases so far, we know that even though the field is only
> one or two bytes, it's actually safe to do a four-byte load/store to it
> because the containing structure is large enough.
>
> I'm not really comfortable with making that a blanket assumption. I'd
> really like to know exactly what we're overwriting when performing 
> these
> hacks^Woperations.
You won't be overwriting anything else since you'll be performing an 
atomic read-modify-write that does not change any of the bits outside 
the subword of interest. More precisely, you may overwrite those bits 
outside the subword, but it's guaranteed to be with the current 
contents so there'll be no externally visible change and you won't 
clobber anything. Since you'll do an aligned longword operation you 
can't end up straddling a page boundary or anything nasty like that.
 -- Keir


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